1. Field of the Invention
The present invention relates to optical leveling modules, and particularly to an optical leveling module which can adjust a light intensity distribution of an image capturing device.
2. Description of Related Art
Image definition, uniformity and trueness are conventionally important characteristics of an optical imaging device such as a digital camera. Commonly, some reduction (e.g., from 40% to 70%) in an image intensity occurs in using a camera because of vignetting or Cos4law. The image intensity at a center thereof is stronger than that at a periphery thereof.
Vignetting is when the part of an optical beam that is directed to the periphery of an image does not completely pass through the effective aperture diameter. In a typical three-lens system, said part of the optical beam is blocked by lens frames in front of and behind the aperture diameter. Cos4law defines that the peripheral image intensity is reduced in proportion to four times the cosine of the incident angle. When forming an image, vignetting can be avoided by reducing the aperture diameter. However, the Cos4law cannot be avoided by reducing the aperture diameter.
Conventionally, three kinds of means are used to adjust or compensate for the distribution of light intensity caused by the Cos4law.
The first means is adjusting the intensity of the light source. A central portion of the light source is shielded proportionately with the known unadjusted reduction in edge intensity of the image. This method is reasonably efficacious. However, it results in too much attenuation of the optical input. In addition, it adds to a height of the lens module. Furthermore, it is rather inconvenient implement.
In the second means, a transversal shelter board is disposed between the object and the image. A height of the shelter board progressively decreases from a center thereof to a periphery thereof. This controls the luminous flux, in order to compensate for the distribution of light intensity caused by the Cos4law. However, the shelter board is ordinarily made of plastic, and is formed by insert molding. It is difficult to accurately control the shape of the shelter board, which is prone to exceed acceptable tolerances. Accordingly, the resulting image after compensating is frequently unsatisfactory.
In the third means, the distribution of light intensity received by a CCD (Charge Coupled Device) image sensor or a CMOS (Complementary Metal-Oxide Semiconductor) image sensor is adjusted using a DSP (Digital Signal Processor). Accordingly, the image as recorded has a uniform distribution of intensity of illumination. However, this method complicates the circuit design of the DSP, and increases costs.
An optical leveling module of an image capturing device is disclosed in China Patent No. 97229889. The disclosure includes a lens module comprising at least one lens, an aperture diaphragm with an optical passing part, and a modulated diaphragm disposed in front of or behind a first lens of the lens module in order to shield a central portion of incoming light. Leveling of the image is attained, but at the expense of too much of the optical input being attenuated. In addition, the modulated diaphragm must be kept some distance from the image sensor; otherwise, no optical input is received by the central part of the image sensor and no image is formed. Furthermore, the form of the modulated diaphragm is limited by the form of the aperture diaphragm. As a result, the optical path can only have a limited range of configurations.
Accordingly, an new optical leveling module is desired to overcome the above problems.